Rare Event: Jupiter to Hide Behind the Moon

Jupiter and its moons as they will appear through a telescope prior to the occultation from some mid-northern latitudes.Credit:

A rare and spectacular event will occur in the early morning hours of Tuesday, Dec. 7 when the brilliant planet Jupiter and three of its largest satellites pass behind Earth's Moon.

Astronomers refer to this phenomenon as an "occultation,"
taken from the Latin occultare,
which means "to conceal." This eye-catching sight will be visible in complete
darkness across all of eastern and much of central North America.

The waning Moon will be 26 percent illuminated, a crescent just 4-? days before
new.

It's been decades since Jupiter has played hide-and-seek for U.S. skywatchers
under favorable dark-sky circumstances. Across the western and central states,
Jupiter was occulted at dawn on Oct. 19, 1968 in a widely watched event. For
viewers farther East the wait has been considerably longer.

The last time New Yorkers were treated to such a favorably placed dark-sky
Jupiter occultation was on Sept. 3, 1889. After 2004, there will not be another
similarly favorable Jupiter occultation for most North Americans until Oct.
6, 2026.

Where it's visible

Earth's Moon and Jupiter are of course very far apart, but they both travel
in roughly the same plane of space, making an occultation possible when the
geometry is just right. What you will see (or not see) depends on location.

Along the Eastern Seaboard, the Moon will be more than 20 degrees
above the horizon when it covers Jupiter in the predawn hours. (Your clinched
fist, held at arm's length, measures approximately 10 degrees in width.) The
Moon will have risen to more than 30 degrees above the east-southeast horizon
by the time Jupiter begins to reappear.

Across the western Great Lakes, the eastern Great Plains and down into the
Deep South for example, the emergence of Jupiter occurs with the Moon about
20 degrees high. Along a line from about central North Dakota down into western
Texas, Jupiter's disappearance will roughly coincide with moonrise, but when
Jupiter is ready to emerge, the Moon will have climbed up to an altitude of
about 10 degrees.

West of this line, viewers will only be able to see Jupiter's reappearance,
with the Moon quite low. Along a line running from just west of Edmonton, Alberta,
south through northern Idaho, down to Nogales, Arizona, Jupiter's emergence
occurs at moonrise. To the west of this line, no occultation will occur.

Out of the 48 contiguous United States, only Washington, Oregon and California
are completely shut out of this sky show, as the main event will have already
taken place before the Moon and Jupiter rise. But even from these localities,
the sight of Jupiter hovering just above and to the right of the lunar crescent
as they rise into full view will still make for a very striking sight.

Texas grazing

A most interesting sight will be seen from a 37-mile-wide strip centered from
roughly Marfa to Matagorda in Texas, then continuing east across the Gulf of
Mexico to Key West, Florida. Within this zone the Moon's mountainous edge will
be silhouetted against Jupiter for several minutes, but the planet will never
be completely covered.

The graze will take place near the crescent's southern cusp.

Map of grazing event.Click
to enlarge

The best views will be seen at the northern edge of the graze zone, where the
partial occultation will last the longest -- about 20 minutes. San Antonio,
Texas is very close to the northern edge; in the Florida Keys, the town of Marathon
is also situated very near to the graze zone's northern limit.

To the south of the graze zone, the Moon will miss Jupiter, skimming just barely
above it.

When to watch

The amount of time that Jupiter will be completely hidden behind the Moon depends
on your location and can range from as little as a few minutes to just over
an hour. This table shows when Jupiter will disappear and reappear from select
cities on Dec. 7, rounded off to the nearest minute. The times are based on
predictions by the International Occultation Timers Association (IOTA). More
detailed information, including maps of the occultation zone and times for more
than 275 cities in North America, is available at the IOTA
website.

Michael Molnar, an astronomer and former
manager of the Physics Instructional Labs at Rutgers University,
believes that the planet Jupiter also holds the key to resolving
one of the world's great mysteries: What might have been the fabled
Star of Bethlehem?

In his book, "The Star of Bethlehem,
The Legacy of the Magi" (Rutgers University Press, 1999), Molnar
says that an ancient Roman coin, minted in the historic city of
Antioch holds the answer. Molnar became intrigued by the image of
a ram looking back at a star.

After doing considerable research using
astronomical, astrological and historical clues, he argues that
the image of the star on the coin was Jupiter, while the ram looking
back at the star was the zodiacal constellation of Aries, a star
pattern that was also the symbol of Judea. "In Roman times,"
he adds, "astrologers claimed that the Moon enhanced the astrological
influences of a planet during close conjunctions."

Ancient astrologers, such as the Magi,
very likely believed that a new king would be born when the Moon
passed in front of Jupiter - an eclipse that would herald the birth
of Christ. And modern-day computer wizardry enabled Molnar to discover
that while Jupiter was in Aries, it was occulted by a crescent Moon
on April 17, 6 B.C.

Molnar believes that the Star of Bethlehem
was not a singular occurrence, but was actually composed of an entire
sequence of events, which also included Jupiter's appearance in
the eastern sky within the sign of Aries. But the 6 B.C. occultation
of Jupiter on April 17 was perhaps the most important component.
He adds: "Writing during the reign of Emperor Constantine,
Firmicus Maternus, a christianized pagan, claimed that these conditions
in Aries gave rise to immortal and divine people! I believe that
he was making a reference to the Star of Bethlehem."

But the upcoming Jupiter occultation
differs from the one in 6 B.C. in that it will not occur in Aries,
but within the constellation Virgo.

Jupiter is bright, shining at magnitude minus 1.8. That's brighter than any
star in the sky on an astronomers' scale in which negative
numbers denote the brightest objects.

You might be able to see Jupiter disappear using no optical aid at all, but
binoculars or a small telescope
will certainly give a much better view. The reappearance of the planet should
be plainly visible to the unaided eye.

With the naked eye, Jupiter appears as a star-like point of light. But with
telescopes and even binoculars it becomes a discernable disk. Observed with
magnification, it will not disappear or reappear in an instant as a star would,
but rather it will take an interval of time to fully occult and emerge

For most locations, it will take the Moon at least a minute to completely cover
or uncover Jupiter. In south-central Texas and south Florida, just to the north
of the partial occultation zone, this process could take up to three minutes
or more.

The most spectacular part of this sky show will be the gradual reappearance
of Jupiter from behind the Moon's dark limb.

The description of Jupiter's gradual reappearance from behind the Moon's dark
limb in 1968 was described as "quite dramatic" as seen from Chicago.
Meanwhile an observer at La Pryor, Texas noted that the most spectacular feature
for him was the naked-eye impression of Jupiter looking "like a brightening
jewel on the dark limb of the Moon."

Also likely to be visible on this morning will
be earthshine,
a faint grayish-blue illumination of the dark part of the Moon's disk.

This dim light is produced by sunlight striking
the "dark" side of the Moon after first being reflected from the Earth's
surface, clouds and atmosphere. The effect will give the Moon an almost 3-D
quality and will enhance the overall spectacle.

Bonus: The Galilean moons

During the occultation, it will be possible to also watch three of Jupiter's
four largest satellites disappear.

The moons will likely be lost in the glare of the bright lunar crescent just
before they disappear behind the Moon, but their emergence from behind the Moon's
dark side should be readily visible through binoculars or a small telescope.

Callisto will emerge from behind the Moon's dark limb about 13 or 14 minutes
before Jupiter itself, followed by Ganymede about 9 or 10 minutes later. Then,
just about one or two minutes after Jupiter has fully reappeared, Europa will
come into view.

Due to the angular sizes of the three Jovian moons, they will take up to a
few seconds to brighten as they move out from behind the Moon. The fourth Galilean
satellite, Io, will be behind Jupiter and not part of the show.

Joe Rao
serves as an instructor and guest lecturer at New York's Hayden Planetarium.
He writes about astronomy for The New York Times and other publications, and
he is also an on-camera meteorologist for News
12 Westchester, New York.

DEFINITIONS

Degrees
measure apparent sizes of objects or distances in the sky, as seen from
our vantage point. The Moon is one-half degree in width. The width of
your fist held at arm's length is about 10 degrees.

Magnitude is
the standard by which astronomers measure the apparent brightness of objects
that appear in the sky. The lower the number, the brighter the object.
The brightest stars in the sky are categorized as zero or first magnitude.
Negative magnitudes are reserved for the most brilliant objects: the brightest
star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest
stars visible under dark skies are around +6.